-4ab-4a²-b² =-（2a+b）²
3ab²+6a²b+3a³ =3a(b²+2ab+a²)=3a(a+b)²
49(2a-3b)²-9(a-b)²=(14a-21b)²-(3a-3b)²=(17a-24b)(11a-18b)